A series of optical brighteners based on stilbene disulfonic acid including those which are the condensation products of 4,4'-diamino-2,2'-disulphonic acid and cyanuric chloride are well known to the art. These products are utilized by dissolving them into an aqueous medium which is then used to treat the appropriate fiber or fabric. Thus, for many years there was very little interest in the appearance of these brighteners. They were simply powdered industrial products but then the manufactureres of household laundry detergents began to include such materials in their final products.
The use of these optical brighteners in finished customer products created an interest in their physical appearance. There was a concern that the presence of such agents in a detergent composition not discolor the composition. Many of these brighteners had a yellow coloration which they imparted to the detergent compositions into which they were incorporated even when the level of incorporation was fairly low. Typically less than about 0.5 wt. % was used.
Others of these brighteners turned yellow after incorporation into an aqueous slurry of the detergent and drying of the slurry. Household detergent compositions were and still are prepared by the alternative techniques of slurrying the components together and drying them or simply dry blending the components together. Thus, the appearance of these optical brighteners both as produced and after drying from an aqueous slurry became of importance.
Also discoloration of a detergent composition from the addition of an optical brightener was of concern for two major reasons. The detergent manufacturers use the physical appearance of the composition as a criterion of quality control. A discoloration from the optical brightener could make it difficult to detect other problems in the composition such as contamination or degradation of another component. Additionally, the consumer often has a negative reaction to a detergent composition which has any yellow coloration. While this is admittedly a physiological factor actually not related to the ultimate performance of the detergent composition in cleaning clothes, it does substantially effect the salability of such products.
It was discovered that both of these discoloration phenomena could be favorably affected by modifying the crystal form of these optical brighteners. Unfortunately but not unexpectedly the conditions necessary to obtain a favorable modification were not universal. It appears that each particular optical brightening compound may require its own unique conditions.
United Kingdom Pat. No. 997,044 discloses the crystal modification of the neutralized form of 4,4'-bis-[2",4"-(diphenylamino)-s-triazinyl-(6")-amino]stilbene-2,2'-disul fonic acid by heating between 100.degree. and 200.degree. C. at between 5 and 225 psig in the presence of an alkaline material. A material which is yellow or has a strong tendency to turn yellow is reportedly converted into a stable white material.
U.S. Pat. No. 3,472,842 discloses the crystal modification of the structurally similar neutralized 4,4'-bis-[2"-phenylamino-4"-(N-methyl-beta-hydroxy-ethylamino)-s-triazinyl -(6")-amino]stilbene-2,2'-disulfonic acid, by heating an aqueous slurry of it containing sufficient electrolyte to keep the optical brightener out of solution to between 100.degree. and 200.degree. C. This patent notes that the procedure of United Kingdom Pat. No. 997,044 is not directly applicable to this compound; in particular it recommends against the presence of the electrolyte which is critical to the procedure of this patent stating it may inhibit the formation of the desired crystal form.
U.S. Pat. No. 3,511,833 discloses an alternate procedure for modifying this same compound by refluxing a slurry of this brightener in an aqueous medium containing both a water soluble organic solvent and sufficient sodium based electrolyte to keep the brightener out of solution. This patent also discloses heating a similar slurry to temperatures as low as 60.degree. C. when seed crystals of the desired crystal form are added.
U.S. Pat. No. 3,925,260 discloses the crystal modification of several sodium N,N'-bis-[4-anilino-6-(2-alkoxyalkyl amino)-1,3,5-triazin-2-yl]-4,4'-diamino stilbene-2,2'-disulphonates by holding an aqueous solution of the particular brightener containing between 1 and 2.5 volume percent of sodium based electrolyte at 20.degree. to 70.degree. C. The solution pH is above 7 and seed crystals are typically added. An alternative procedure similar to U.S. Pat. No. 3,511,833 is also disclosed; heating of the brightener in an aqueous medium containing table salt and an organic solvent.
U.S. Pat. No. 3,994,834 discloses that 4,4'-bis-[4-phenyl-v-triazolyl-(2)]-stilbene-2,2'-disulfonic acid must be converted to the potassium salt form to obtain suitable crystal forms. The suitable crystal forms are obtained by heating to temperatures between 50.degree. and 150.degree. C. in the presence or absence of an aqueous alcohol.
U.K. Pat. No. 1,293,804 is concerned with the crystal modification of sodium 4,4'-bis-[4-anilino-6-(bis(2-hydroxyethyl)-amino)-1,3,5-triazin-2-y l]-amino-stilbene-2,2'-disulfonate by heating it in an aqueous medium containing an organic solvent and a sodium based electrolyte. The procedure exemplified utilizes seed crystals and refluxes the water solvent mixture. No indication is given of the necessary temperature for a seed free process.
U.K. Pat. No. 773,152 is concerned with the production of the same optical brightener as U.S. Pat. Nos. 3,472,842 and 3,511,833 but it obtains the yellow colored crystal form which these later patents disfavor. Its preparative technique exposes the sodium salt to temperatures of 90.degree. to 95.degree. C. during the final synthesis step. It is not clear if the compound is in solution at this point.
U.K. Pat. No. 1,173,806 discloses forming a number of optical brighteners of low water solubility into hollow beads. The beads are formed by spray drying an aqueous slurry of the optical brightener using outlet temperatures between 65.degree. and 90.degree. C. Among the suitable brighteners listed is the free acid form of the compound with which the present invention is concerned; 4,4'-bis-[2-anilino-4-methylaminotriazinyl-6-amino]-stilbene-2,2'-disulpho nic acid. Presumably, the sodium salt is not listed because it has a rather high water solubility.
Japanese Patent Publication No. 5880/59 discloses the preparation of the precise compound with which the present invention is concerned. It isolates the sodium salt form of this optical brightener after exposing it to a temperature of about 70.degree. C. during the last synthesis step. It is not clear whether this compound is in solution or slurry at this point. Nothing is said about the crystal form of the recovered product.
The commercial production of the free acid form of this optical brightener has been conducted in the United States. The synthesis involved the exposure of the sodium salt form to temperatures in excesses of 90.degree. C. but this salt was never isolated on a commercial basis because its high water solubility would cause unacceptable yield losses on filtering.